Heaters useful for heating of subterranean formations are disclosed in, for example, U.S. Pat. Nos. 5,060,287, and 5,404,952. Heating of soil or rock formations can be useful, for example, for recovery of hydrocarbons or removal of contaminates from soil. Control systems for electrical heaters such as these are discussed in, for example, U.S. Pat. No. 5,189,283, which discloses a control scheme for controlling a current control to a power control with a smooth cross-over control. U.S. Pat. No. 5,164,660 discloses a related method to measure power and current to such a heater.
Power to heaters such as these are advantageously powered by a zero-fired control such as those suggested in U.S. Pat. No. 4,777,350. A zero-fired control scheme passes full cycles of power, and then does not pass power for full cycles to a heater element, with the opening and closing of the electrical circuit occurring when the alternating power passes through neutral potential. U.S. Pat. No. 5,396,047 suggests an improvement to opening and closing the power circuit when the potential passes through the ground potential, the improvement being that at these times, power is switched between heater elements. Power therefore does not cut in and out, but only goes to a different element.
It is also known to transmit control signals through power lines as disclosed in, for example, U.S. Pat. No. 5,361,982.
Although there is considerable art available that is applicable to control of electrical heaters such as those of the present invention, an efficient and cost effective control system is still needed. It is therefore an object of the present invention to provide an efficient and cost effective control system for parallel heater elements.